Can i use a 2009 iMac 27 as a monitor for a Mac Mini M1 2020?

Use your iMac as a display with target display mode - Apple Support

(The "Mac from 2019 or earlier" is for Thunderrbolt iMacs, and doesn't apply to your Mini DisplayPort iMac.)

You will need a USB-C to Mini DisplayPort cable, such as:

mophie USB-C Cable with Mini DisplayPort Connector - Apple (CA)

Yes, you can as a target display mode assuming your iMac is running an older OSX.

Check this link on a very informative options:

https://forums.macrumors.com/threads/how-to-integrate-m1-mac-mini-with-target-display-mode-thunderbolt-displays-up-to-3-displays.2274691/